This invention relates to a fuel supply control system for internal combustion engines, and more particularly to a deceleration fuel cut device provided in a fuel supply control system of this kind, for performing a fuel cut operation at engine deceleration.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel quantity metering or adjusting means for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
On the other hand, in these days there is a tendency for automobile fuel cost to gradually increase. To cope with this tendency, it has conventionally been employed to cut off the supply of fuel to the engine at engine deceleration, for reduction of the fuel consumption. Detection of a decelerating condition of the engine for carrying out the fuel cut is conventionally made on the basis of the opening of a throttle valve in the intake pipe of the engine, and when the throttle valve opening is decreased below a predetermined opening (almost equal to full closing of the valve) and simultaneously the engine rotational speed is higher than a predetermined rotational speed, the fuel cut is carried out.
To detect the opening of the throttle valve, generally used is a potentiometer which is connected to the valve body of the throttle valve or a sensor adapted to detect negative pressure in the intake pipe of the engine through a negative pressure intake port arranged to open in the intake pipe at a location slightly upstream of the throttle valve in its full closing position. However, it is very difficult to accurately detect the opening of the throttle valve by means of the above type sensors or the like, when the throttle valve is in almost full closing position, thus making it difficult to carry out a proper fuel cut operation.
On the other hand, if the intake pipe pressure at which the fuel cut is to be effected is too low, the engine can be stalled upon disengagement of the clutch, and the driveability of the engine can be spoiled at rapid acceleration of the engine, when the engine returns into a normal operating condition after termination of the fuel cut. Particularly, if the fuel cut effecting intake pipe pressure is too low, unburned fuel can be emitted in large quantities together with exhaust gases, which reacts with a three-way catalyst arranged in the exhaust pipe of the engine to cause burning of the catalyst, resulting in emission of detrimental exhaust gases.
Further, if the fuel cut is carried out when the engine temperature is low, the engine can also be stalled upon disengagement of the clutch immediately after termination of the fuel cut, since sliding component parts of the engine have large frictional resistance in such a cold condition.